Well, when EA moved to their own platform I moved with them purchasing DA:O. Thing is, I haven't purchased a single game from them since then. Not because I can't stand a different platform, but because I simply forget about those games. I would gladly play ME3 and probably DA:I too, but when I'm not exposed to those games on Steam I need a much bigger incentive than "they might be fun".
I do believe competition is healthy for a market, and Steam being so big while taking such a large cut might be problematic in the long run. It's damn convenient though.
I'm not interested in FO76, but I'm pretty sure I'll buy the next TES game although Skyrim isn't a favourite of mine, and the little I know about Starfield makes me interested in that as well. If I have to use a different platform than Steam I will, and I'm guessing most Beth fans feel the same way. But the exposure to new players will be a lot lower without Steam.
You're vastly overestimating the power Steam exposure has for huge AAA titles.
Steam exposure is vital for indie games - or games that don't have much in the way of marketing.
Publishers like EA or Bethesda (ZeniMax) make most of their money on consoles, first of all. Secondly, they have nearly infinite marketing budgets.
EA is a business - nothing more, nothing less. In that way, I give you my personal guarentee that if the suits found themselves losing money by not being on Steam - they would move back to Steam.
Very, very few people in this world are only exposed to Steam and nothing else. Most people go on Facebook, Youtube - or whatever. The VAST majority.
There's no exclusive little bubble called "Steam marketing".
So, if the game is marketed on those sites - it doesn't have to be marketed on Steam for people to be aware of it.
Furthermore, FO76 has already been marketed and hyped - and most people potentially interested ALREADY know about it. They don't need Steam to tell them it exists.
It's incredibly simple.
As for Bethesda.net - it's too soon to say anything for sure.
FO76 is a new thing for them. Some people seem to think they're exclusively targeting fans of singleplayer games. Obviously, they're going for an entirely new audience ON TOP of their established fanbase.
One thing is for sure, with 20+ million copies sold of their more popular games - there will be enough potential customers to make it sustainable. But whether it's a "hit" or a big smash is anyone's guess.
Since it's their first outing in this genre - which I suppose is something of a hybrid Fallout/MMO-lite looter shooter - I'm sceptical they really know what they're doing.
As such, I would be careful expecting big numbers - and I don't know if they are.
I think, for them, it's more a way of testing the waters - before they bet on MP in a bigger way.
But it's not a small project by any means. If you watch the making of documentaries - it's clear that this is as big a thing as anything they put out.